Solar cells are semiconductor workpieces that include emitter regions, which may be p-type doped, and surface fields, which may be n-type doped. Solar cells utilize a pn junction, created between the emitter regions and the surface fields, to generate electrical current in the presence of photons. To carry this produced current from the workpiece, contact regions are disposed on the surface of the workpiece. These contact regions are exposed areas of doped semiconductor, and are used to electrically connect the semiconductor structures, which are contained within the workpiece, to the exterior of the workpiece. In some high efficiency solar cells, such as interdigitated back contact (IBC) solar cells, the contact regions for the emitter regions and the surface fields are located on one surface of the workpiece. In other solar cell structures, the contact regions of the emitter region and surface fields may be located on two opposing surfaces of the workpiece.
The manufacture of high efficiency solar cells has many, often conflicting, requirements. For example, screen printed contacts typically require specific dopant profiles for proper solar cell performance. These contacts require high interstitial dopant concentrations at the surface of the substrate. Current solar cell manufacturing methods may use fired paste contacts with a single dopant, such as phosphorus, to achieve this high concentration. However, achieving a high surface concentration may require a high concentration of the dopant to be introduced throughout the emitter of the solar cell. This high concentration of dopant throughout the emitter increases carrier recombination and, consequently, may lower cell efficiency. Thus, contacts between the emitter and the contacts are improved by introducing high dopant concentrations near the contact regions. However, the introduction of this dopant throughout the emitter degrades solar cell performance.
Another issue is that recombination of electron-hole (e-h) pairs at the surface of a solar cell typically limits solar cell efficiency. This recombination can be reduced by repelling minority carriers from the surface of the solar cell. One way to repel minority carriers is to put a shallow, high concentration layer of dopant at the surface of the solar cell. This layer needs to remain in place throughout any subsequent thermal processing. However, previous methods would diffuse this dopant layer throughout the emitter, reducing its effectiveness.
While the above criteria require a shallow, high concentration at the surface of the substrate, other criteria may require a deeper dopant concentration. One example of such a solar cell criteria is a p-n junction. Efficiency is enhanced if the p-n junction is located deep within the substrate, away from the surface of the substrate. The presence of a deep dopant profile also may lower series resistance of the solar cell. This deeper dopant concentration has been previously performed using a high-diffusivity dopant. Such a high-diffusivity dopant may not allow high concentration of the dopant at the surface of the cell without introducing an excessive number of dopant atoms into the silicon, thereby increasing recombination.
Thus, current solar cell design is limited by the dopant profiles that can be achieved by diffusing dopants into the silicon of the solar cell. Accordingly, there is a need in the art for improved dopant profiles for solar cells and, more particularly, a method that simply and cost-effectively creates a dopant profile optimized for solar cells.